A. Field of the Invention
This invention relates to microcapsules and to methods for making and using said microcapsules. In particular, this invention relates to microcapsules comprising encapsulated droplets of a liquid material that is substantially insoluble in water, where the encapsulating agent is a film formed from a polymer that includes one component that is photoactivatable. Photoactivation of the microcapsules results in partial or substantial loss of encapsulation properties of the encapsulating polymer and concomitant partial or substantial release of the encapsulated material following illumination of the microcapsules with light, including sunlight and ambient indoor light.
B. Description of the Related Art
The use of membranes, coatings, and capsules for the controlled release of liquid materials is well known in the chemical arts for both agricultural and non-agricultural chemicals. In agriculture, controlled-release techniques have improved the efficiency of herbicides, insecticides, fungicides, bactericides, and fertilizers. Non-agricultural uses include encapsulated dyes, inks, pharmaceuticals, flavoring agents, and fragrances.
The most common forms of controlled-release materials are coated droplets or microcapsules, which are coated solids including both porous and non-porous particles, and coated aggregates of solid particles. In some instances, a water-soluble encapsulating film is desired, which releases the encapsulated material when the capsule is placed in contact with water. Other coatings are designed to release the entrapped material when the coating is ruptured by external force.
Still further coatings are porous in nature and release the entrapped material to the surrounding medium by diffusion through the pores, typically at a slow rate. In addition to providing controlled release, such coatings also serve to facilitate the dispersion of water-immiscible liquids into water and water-containing media such as wet soil. Droplets encapsulated in this manner are particularly useful in agriculture, where water from irrigation, rain, and water sprays is frequently present A variety of processes for producing such capsules are known in the art.
U.S. Pat. Nos. 2,800,457 (Green et al., Jul. 23, 1957) and 2,800,458 (Green, Jul. 23, 1957) describe formation of capsules by phase separation from an aqueous solution through the coacervation of a hydrophilic colloid sol.
U.S. Pat. Nos. 4,046,741 (Scher, Sep. 6, 1977) and 4,140,516 (Scher, Feb. 20, 1979) disclose an interfacial polymerization process, whereby the film-forming reactants are dissolved in the hydrophobic liquid which is dispersed in water, the reaction occurring at the interface when the phases are placed in contact as an emulsion.
U.S. Pat. No. 3,726,804 (Matsukawa et al., Apr. 10, 1973) describes another interfacial polymerization process, whereby all the film-forming ingredients initially reside in hydrophobic droplets which also contain a low boiling point or polar solvent in addition to the material to be encapsulated. Upon heating, the solvent is released into the aqueous phase (the continuous phase of the emulsion), and the film-forming materials accumulate at the interface and polymerize.
Olefin polymerization using a peroxide catalyst is described in Japanese patent publication No. 9168/1961, whereby an oil-insoluble polymer is formed at the surfaces of oil drops.
British Patent Nos. 952,807 and 965,074 describe a process whereby a solid such as wax or a thermoplastic resin is melted, dispersed and cooled to form an encapsulating film around liquid droplets.
U.S. Pat. No. 3,111,407 (Lindquist et al., Nov. 19, 1963) describes a spray-drying method which form-s encapsulated droplets at the instant of atomization.
These processes vary in terms of equipment expense, energy requirements, ease of controlling microcapsule size, the need for extra reagents such as catalysts and settling agents, and percent microcapsule phase. In addition, the type of prepolymers utilized in these processes results in different release properties of the microencapsulated materials. Moreover, the art teaches microcapsules that release their contents under restrictive conditions (contact with water or mechanical rupture) that are not appropriate or advantageous for all applications. There is therefore a need in the art for microcapsules and methods for producing microcapsules that release the entrapped material under less restrictive conditions, particularly upon simple exposure to electromagnetic radiation, preferably common sunlight and/or room light.